Raman spectroscopy is a powerful and effective tool for analytical studies of biological and chemical samples. Raman scattering is inelastic light scattering from a sample that may yield a molecular fingerprint of the constituent molecules. However, an inherent limitation of this technique is the low Raman cross section of bio-molecules. Hence, long integration times are required to obtain a good signal to noise ratio. Nevertheless, Raman spectra have rich information content and a single Raman spectrum can provide information about all the molecular constituents of the sample.
Raman spectroscopy has numerous areas of application. For example, it has been used to analyse whisky, see for example A. Nordon, A. Sills, R. T. Burn, F. M. Cusick, and D. Littlejohn “Comparison of non-invasive NIR and Raman spectrometries for determination of alcohol content of spirits,” Analytica Chimica Acta 548, 148-158 (2005). This study compares NIR spectroscopy with Raman spectroscopy, and concludes that Raman spectroscopy performs better for concentration calibration. However, the authors raise concerns about implementing a laser based Raman spectroscopic detection technique in a production line using free-space Raman detection devices. Also typical Raman acquisition times range from 10s of seconds to several minutes and require sample volumes in the range of milliliters.
Raman spectroscopy has been combined with microfluidic systems. To overcome the limitation of the inherently low Raman cross section, Surface Enhanced Raman Spectroscopy (SERS) based detection schemes have been employed in microfluidic systems. Other experiments have used confocal Raman microscopy for online monitoring of chemical reactions. In all these applications, monitoring is performed using a bulk Raman microscope and a microfluidic chip. However, using microscope based systems to collect Raman data from microfluidic chips can cause problems, because the signal is acquired through a substrate which has its own background signal. This limits the detection efficiency of the system. Using a microscope also precludes miniaturization.
There are two major issues when Raman spectroscopy is used in microfluidics: intensity of the Raman spectra and the background from the substrate of microfluidic chip. The former issue can be addressed using long acquisition times, SERS based detection approaches etc. The latter issue can be addressed by using a fused silica glass based microfluidic chip, which would have reduced fluorescence background and no Raman peaks in the fingerprint region. Alternatively, confocal detection schemes could be used to avoid background from the substrate. However, this results in comparatively higher acquisition time ranging from 10s of seconds to several minutes.
The article “Fibre Probe Based Microfluidic Raman Spectroscopy” by Ashok et al, Optics Express, Vol. 18, No. 8, 29 Mar. 2010 describes a fiber probe embedded in fluidic channels for Raman detection. This method allows in situ probing of an analyte in a fluidic channel, fabricated using microfluidic fabrication techniques, and so avoids any background from the substrate in the collected Raman spectra. However, the fluidic channel size of such devices is in the millimeter scale, and so it is not feasible to integrate this detection scheme in microfluidic chips whose channel dimensions are in micrometer scales.